PDF《西藏甲玛—驱龙地区叶巴岩组构造学特征》

76 地球学报第三十四卷 nappe tectonic system;

④ in Yeba tectonite group, kinematic direction of two-stages of ductile deformation is on the opposite: in the first stage, the upper layer shearing was from south to the north;

in the second stage, the upper layer shearing was from north to south. The second ductile deformation and later multi-stage ductile-brittle deformation had multi-stage progressive deformation characteristics, and after southward nappe-fold lifting to the superficial level, Yeba tectonite group also experienced multi-stage brittle-ductile deformation, especially at the position near the main napping plane of Jiama(Gyama)-Kajunguo thrust-gliding nappe tectonic system;

⑤ the first stage ductile deformation of Yeba tectonite group occurred around 94C85 Ma, belonging to Late Cretaceous Turonian-Coniacian period and related to back-arc rift stretching that resulted from northward subduction of the Brahmaputra oceanic crust;

the second stage ductile deformation and the later main brittle-ductile deformation occurred around

50 Ma in Eocene, related to Indo-Asia collision, and the post collision tectonism resulted in the superposition of multi-stage brittle-ductile deformation. Key words: Jiama(Gyama)-Qulong area in Tibet;

Yeba tectonite group;

deformation;

kinematics;

dynamics 甲玛矿床和驱龙矿床是近年冈底斯矿产勘查突 破的两个重要矿床, 两者相距不远, 以叶巴岩组火 山沉积岩系相联系, 叶巴岩组的构造特征直接关系 到两矿床关系的认识.前人对叶巴岩组研究多侧重 于其形成的构造环境(耿全如等, 2005, 2006;

朱弟成 等, 2008), 对其变形特征的研究则较薄弱.1/5 万及 1/25 万区域地质调查等(西藏自治区地质调查院, 2006, 2007)加强了变形构造研究, 认为区域性的谢 通门―驽玛―沃卡韧性剪切带经过了驱龙矿区南侧 (蒋光武等, 2002), 并在叶巴岩组中厘定数条韧性剪 切带. 作者通过野外路线地质调查和室内显微结果观 察, 对叶巴岩组变形、运动学、动力学进行了研究, 初步勾勒出叶巴岩组的构造变形特征, 构建驱龙、 甲玛两矿床的成岩成矿的构造地质学联系, 为进一 步探讨驱龙、甲玛这对拉萨经济圈重要的矿业双子 星(唐菊兴等, 2010b)的内在联系奠定基础.

1 区域地质简况 研究区处于南冈底斯弧盆系中北部(图1), 侏罗 纪至白垩纪为弧后裂谷、随后经历了碰撞造山、陆 内伸展等演化阶段. 伴随雅鲁藏布江洋俯冲, 研究区所在区域处于 弧后裂谷盆地, 先形成中下侏罗统叶巴岩组(J1-2y)双 峰式火山沉积岩系(>

17000 m), 其后相继形成上侏 罗统碎屑碳酸盐岩系(厚1700 余m), 白垩系碎屑岩 系(>

4100 m). 伴随雅鲁藏布江洋的关闭, 研究区所在区域发 生推覆叠缩, 形成推覆-滑覆构造系.其后发生大规 模中酸性岩浆侵位(45 Ma±). 图1研究区大地构造位置图 Fig.

1 Tectonic location of the study area 碰撞后至中新世, 研究区发生多期次中酸性岩 浆侵入活动, 但强度明显减弱, 并以中新世岩浆活 动最为广泛, 多呈岩株、岩枝、岩脉. 甲玛、驱龙矿床的形成与上述构造岩浆活动有 密切关系.甲玛矿床位于叶巴岩组分布区南侧, 产 在以叶巴岩组为准原地系统的甲玛卡军果推覆构造 系前部带;

驱龙矿床则产于叶巴岩组分布区南边 部.

2 叶巴岩组构造岩系 在甲玛―驱龙地区南北宽约

16 km, 东西长约